1. Field of the Invention
The present invention relates to a method for producing nano-carbon materials. More particularly, the present invention relates to a method, which enables one to quantitatively produce nano-carbon materials from a relatively inexpensive starting material in a simple manner using a relatively inexpensive reaction apparatus. The nano-carbon materials produced according to the present invention include carbon nanotubes, carbon nanofibers, and the like.
2. Related Background Art
A fullerene C60 having a soccer ball-like polyhedral molecular structure comprising 60 carbon atoms was discovered by H. W. Kroto, R. E. Smallry and R. F. Curl in 1985. Following the discovery of the fullerene C60, a carbon nanotube (CNT) corresponding to a cylindrical molecule of a fullerene, specifically having a molecular structure in which a graphene sheet (a single atomic layer of crystalline graphite) rolled up into a cylinder, was discovered by lijima in 1991. Since then, other carbon nanotubes have been discovered, and various studies on their industrial application have been carried out.
It has also been reported that these carbon nanotubes have an excellent field emission performance, a function to take up and store lithium therein and release said lithium in the electrochemical reaction, a large specific surface area and a good conductivity. In view of this, researches have been conducted regarding the use of these carbon nanotubes as electrode materials in FEDs (field emission displays), as electrode materials in rechargeable lithium batteries, as catalyst-retaining materials in fuel cells in which polymer solid electrolytes are used, and as hydrogen storage materials in hydrogen storage systems.
As the method for producing such carbon nanotubes, there are known a method wherein arc-discharge is generated in an gas atmosphere containing a carbon material such as hydrocarbon, a method wherein a target comprising graphite is evaporated by irradiating a laser thereto, and a method wherein a gaseous carbon material comprising acetylene or the like is subjected to thermal decomposition on a substrate having a catalyst of cobalt metal or nickel metal arranged thereon.
Particularly, Japanese Laid-open Patent Publication 6(1994) -157016 (hereinafter referred to as “Patent Document 1”) discloses a method for producing carbon nanotubes, wherein arc-discharge is generated between a pair of carbon rods respectively as a positive electrode and a negative electrode in an inert gas atmosphere to deposit a carbon nanotubes-containing solid material on the negative electrode.
Japanese Laid-open Patent Publication P2000-95509A (hereinafter referred to as “Patent Document 2”) discloses a method for producing carbon nanotubes, wherein a rod-shaped positive electrode containing carbon and non-magnetic transition metal and a rod-shaped negative electrode comprising graphite are arranged such that their tips are opposed to each other and arc-discharge is generated between the tip of the positive electrode and that of the negative electrode in an inert gas atmosphere to deposit carbon nanotubes on the tip portion of the negative electrode.
Japanese Laid-open Patent Publication 9(1997) -188509 (hereinafter referred to as “Patent Document 3”) discloses a method for producing carbon nanotubes, wherein a carbon material and a metal catalyst are supplied into a high frequency plasma generated to deposit carbon nanotubes on a substrate.
Japanese Laid-open Patent Publication 10(1998) -273308 (hereinafter referred to as “Patent Document 4”) discloses a method for producing carbon nanotubes, wherein a graphite-containing carbon rod is positioned in a quartz tube arranged in an electric furnace and laser light is irradiated onto the carbon rod in an inert gas atmosphere to deposit carbon nanotubes on the inner wall face of the quartz tube.
Japanese Laid-open Patent Publication P2000-86217A (hereinafter referred to as “Patent Document 5”) discloses a method for producing carbon nanotubes, wherein gaseous hydrocarbon is thermally decomposed on a catalyst comprising a molybdenum metal or a molybdenum metal-containing material to deposit carbon nanotubes on said catalyst.
Separately, Carbon Vol. 36, No. 7-8, pp. 937-942, 1998 (Yury G. Gogotsi et al.) [hereinafter referred to as “Non-patent Document 1”] describes a method wherein filamentous carbons are formed from paraformaldehyde by way of hydrothermal reaction at a temperature of 700 to 750° C. under 100 MPa pressure for 150 hours.
Journal of Materials Research Society, Vol. 15, No. 12, pp. 2591-2594, 2000 (Yury Gogotsi et al.) [hereinafter referred to as “Non-patent Document 2”] describes a method wherein multi-wall carbon nanotubes are formed from polyethylene by way of pyrolysis of said polyethylene in the presence of nickel at a temperature of 700 to 800° C. under 100 MPa.
Journal of American Chemical Society Vol. 123, No. 4, pp. 741-742, 2001 (Jose Maria Calderon et al.) [hereinafter referred to as “Non-patent Document 3”] describes a method wherein multi-wall carbon nanotubes are formed from amorphous carbon through a hydrothermal treatment of the amorphous carbon in the absence of a metal catalyst at a temperature of 800° C. under 100 MPa pressure for 48 hours.
However, the methods disclosed in patent Documents 1 to 5 have disadvantages such that the starting material and the apparatus used for practicing the method are costly and therefore a product obtained becomes unavoidably costly and it is difficult to quantitatively produce nano-carbon materials.
Similarly, the methods described in Non-patent Documents 1 to 3 have disadvantages in that because high pressure of 100 MPa is used, a specific high pressure capsule made of Au (which is costly) and which can withstand such high pressure is employed as the reaction vessel, and the starting material and water are introduced into the capsule, wherein the starting material is subjected to a hydrothermal reaction at a high temperature (700 to 800° C.) under a high 100 MPa pressure. Therefore, a product obtained by any of these methods unavoidably becomes costly. Non-patent Documents 1 to 3 do not even suggest a method in which nano-carbon materials (including carbon nanotubes or filamentous carbons) can be formed under low pressure condition of less than 60 MPa, using a relatively inexpensive pressure reaction vessel without the necessity of using an expensive pressure reaction vessel.
There is a demand for providing a method capable of quantitatively producing nano-carbon materials (including carbon nanotubes and carbon nanofibers) at a reasonable production cost from relatively inexpensive raw material in a simple manner under low pressure, which does not require the use of a specific and costly pressure reaction vessel as described above.
Separately, Carbon Vol. 40, pp. 2961-2973, 2002 (Mingwang Shao et al.) [hereinafter referred to as “Non-patent Document 4”] describes a method wherein carbon nanotubes having a multi-layered structure are formed by reacting benzene as a starting material with a nickel-iron powder as a catalyst at a temperature of 480° C. under a reaction pressure of about 15 Pa pressure for 12 hours in a stainless steel autoclave. Although this method is advantageous in that the reaction pressure upon forming a nano-carbon material (a carbon nanotube having a multi-layered structure) is low (about 15 Pa), the method has a disadvantage in that the amount of a nano-carbon material produced in the reaction after a relatively long time (12 hours) is small. In particular, a ratio of the weight of the nano-carbon material produced to the weight of the catalyst used is 4.2, which is small. In addition, according to the results of the measured Raman spectrum of the obtained nano-carbon material, which results are described in Non-patent Document 4, it is understood that for the relative intensity between the so-called G-band peak near 1590 cm−1 due to a lattice vibration in the hexagonal lattice network of the carbon atoms and the so-called D-band peak near 1350 cm−1 due to a lattice defect of the carbon atoms, the intensity of the peak near 1350 cm−1 is stronger. This means that the nano-carbon material obtained by the method described in Non-patent Document 4 is accompanied by a number of lattice defects. Thus, although the method described in Non-patent Document 4 has an advantage in that the reaction pressure upon forming a nano-carbon material is low, it is difficult to stably produce a nano-carbon material with only a slight lattice defect. It is desirable to improve the method described in Non-patent Document 4 so that it can stably produce a high-quality nano-carbon material with few lattice defects in a shortened reaction time.
In addition, such nano-carbon materials as described above are expected to be widely used in various technical fields, for instance, as electrode materials in FEDs (field emission displays), as electrode materials in rechargeable lithium batteries, and as catalyst-retaining materials in fuel cells. In view of this, there is a demand for a technique, which can enable one to efficiently produce high-quality nano-carbon materials at a reasonable production cost.